The use of a thick printed circuit board (PCB) with high layer counts means that signals travel longer distances to make connections from one layer to another. Signals are susceptible to longer distances of impedance discontinuity. Discontinuity can deteriorate quality of high-speed signals because of signal reflection, attenuation, and other degradation phenomena.
Signal traces in different PCB layers are generally connected by formation of conductive vias extending through the board. Thick boards can be heavily-populated with components and devices with layers that contain intricate, highly-precise traces with fine detail. Vias connecting the traces can similar be highly intricate and tightly-specified. Drilling of small and intricate tightly-specified vias in the thick printed circuit boards is difficult due to various considerations including variation of two-dimensional registration of structures on the different layers, the increased likelihood of drill bit breakage in the increased thickness boards, and others. Usage of vias to transfer signals across different layers adds signal impedance discontinuity, potentially degrading the signal.
Impedance discontinuity problems can become highly significant for high-speed signals that are transferred from one layer to another in a thick printed circuit board. For example, particular difficulties may arise for bladed computers where a very large number of computers are interconnected, either directly or through a network fabric. A high speed interconnect is generally used to connect the multiple computers. The high speed interconnect extends from an individual computer to an interconnect switch through a backplane. The backplane is typically thick to enable multiple routing connections and supply sufficient mechanical strength. The interconnect signals can be moderately high-speed signals with little tolerance for impedance discontinuity and other signal degradation and attenuation occurring in signal transfer through the vias.